1) Field of the Invention
The present invention relates to a signal transmission system in which an AM (Amplitude Modulation, Intensity Modulation) signal or a QAM (Quadrature Amplitude Modulation) signal is transmitted in a state converted into an FM (Frequency Modulation) signal or a PM (Phase Modulation) signal, which in turn, is again converted into the original (former) AM signal or QAM signal on the receive side.
2) Description of the Related Art
Referring to FIG. 3, a description will be given hereinbelow of one example of a conventional signal transmission system based on optical transmission.
In FIG. 3, a signal transmission system is shown with a transmission section 10, an optical fiber (transmission line) 20 and a receive section 30.
The transmission section 10 comprises an FM modulator 11 (modulating means), a first amplifying circuit 12 (first amplifying means) and a semiconductor laser 13 (light-emitting device), located in the order from the upstream side to the downstream side. The receive section 30 comprises a light-receiving device 31, a second amplifying circuit 32 (second amplifying means) and an FM demodulator 33 (demodulating means), located in the order from the upstream side to the downstream side.
In the signal transmission system thus arranged, the FM modulator 11 of the transmission section 10 first converts an AM signal into an FM signal. In this case, the AM signal includes, for example, frequency-division-multiplexed multi-channel video signals.
The first amplifying circuit 12 amplifies this FM signal, and the semiconductor later 13 converts the amplified FM signal from the electric signal form into an optical signal from. The FM signal forming an optical signal arrives through the optical fiber 20 at the receive section 30. In the receive section 30, the light-receiving device 31 converts the optical FM signal into an electric signal. Following this, the second amplifying circuit 32 amplifies the electric FM signal, and the FM demodulator 33 demodulates the amplified FM signal into the original AM signal.
The FM demodulator 33 is a delaying type FM demodulator. Concretely, the FM demodulator 33 detects the rise (or fall) of an FM signal to generate a pulse with constant width, as an output signal, a voltage corresponding to the pulse density of this pulse. Since the amplitude (voltage) of this output signal is proportional to the frequency of an FM signal, the FM demodulator 33 converts the FM signal into the original AM signal.
In the foregoing signal transmission system, in the case of transmission of a broadband FM signal, a group delay deviation according to frequency arises in the first amplifying circuit 12, the semiconductor laser 13, the light-receiving device 31 and the second amplifying circuit 32. When passing through these components, a phase delay of an FM signal occurs according to frequency. In consequence, at the FM demodulation, the timing of the rise (or fall) of an FM signal shifts to develop a distortion of an AM signal obtained through the demodulation, thus causing degradation of transmission quality.
Also, this problem arises in the case of the modulation of an AM signal into a PM signal and subsequent demodulation of the PM signal into the original AM signal, and for the modulation of a QAM signal, instead of the AM signal, into an FM signal or PM signal and subsequent demodulation into the original QAM signal.
Accordingly, it is therefore an object of the present invention to provide a signal transmission system capable of suppressing the distortion of a demodulated AM signal or QAM signal with simple configuration.
For this purpose, in accordance with a first aspect of the present invention, there is provided a signal transmission system comprising modulating means for modulating one of an AM signal and a QAM signal into one of an FM signal and a PM signal, demodulating means for demodulating the FM signal or the PM signal into the original AM signal or QAM signal, group delay deviation producing means placed between the modulating means and the demodulating means for producing a group delay deviation on the FM signal or the PM signal, harmonic producing means placed on the upstream side of the demodulating means for producing a harmonic of the FM signal or PM signal to superimpose the harmonic on the FM signal or the PM signal, and harmonic phase shifting means placed on the upstream side of the demodulating means for shifting a phase of the harmonic from the harmonic producing means to compensate the group delay deviation.
Thus, a harmonic is superimposed on an FM signal or PM signal and the phase of this harmonic is shifted, which suppress the distortion of a demodulated AM signal or QAM signal.
In this signal transmission system, it is also appropriate that the aforesaid harmonic phase shifting means has a function to adjust a harmonic phase shifting quantity. This can further suppress the distortion of the AM signal or QAM signal by adjusting the harmonic phase shift.
In accordance with another aspect of the present invention, there is provided a signal transmission system comprising modulating means for modulating one of an AM signal and a QAM signal into one of an FM signal and a PM signal, demodulating means for demodulating the FM signal or the PM signal into the original AM signal or QAM signal, group delay deviation producing means placed between the modulating means and the demodulating means for producing a group delay deviation on the FM signal or the PM signal, harmonic producing means placed on the upstream side of the demodulating means for producing a harmonic of the FM signal or PM signal to superimpose the harmonic on the FM signal or the PM signal, and harmonic level changing means placed on the upstream side of the demodulating means for changing a level of the harmonic from the harmonic producing means to compensate the group delay deviation.
This also exhibits a signal distortion suppressing effect similar to that of the first-mentioned signal transmission system by means of the change of harmonic level.
In this signal transmission system, it is also appropriate that the harmonic level changing means has a function to adjust a harmonic level.
In these signal transmission systems, it is also appropriate that the modulating means functions additionally as the harmonic producing means. In this case, since the modulating means also acts as the harmonic producing means, the simplification of the circuit arrangement becomes feasible.
In addition, it is also appropriate that the modulating means is designed to generate a pulsed FM signal or PM signal. In this case, since the modulating means generates a pulsed output, the harmonic is easily attainable.
Furthermore, in these signal transmission systems, first amplifying means, a light-emitting device for converting an electric signal into an optical signal, a light-receiving device for converting an optical signal into an electric signal and a second amplifying means are provided in sequence from the modulating means to the demodulating means, and at least one of the first amplifying means, the light-emitting device, the light-receiving device and the second amplifying means serves as the group delay deviation producing means, while at least one of the first amplifying means, the light-emitting device, the light-receiving device and the second amplifying means serves as the harmonic producing means.
In this case, the simplification of the circuit arrangement becomes feasible because a component to be usually employed for optical transmission also functions as the harmonic producing means.
Still furthermore, in the first-mentioned signal transmission system, first amplifying means, a light-emitting device for converting an electric signal into an optical signal, a light-receiving device for converting an optical signal into an electric signal and a second amplifying means are provided in sequence from the modulating means to the demodulating means, and at least one of the first amplifying means, the light-emitting device, the light-receiving device and the second amplifying means serves as the group delay deviation producing means, while at least one of the first amplifying means, the light-emitting device, the light-receiving device and the second amplifying means serves as the harmonic phase shifting means.
Likewise, in this case, the simplification of the circuit arrangement becomes feasible because a component to be usually employed for optical transmission also functions as the harmonic phase shifting means.
Furthermore, according to the present invention, in a signal transmission apparatus for transmitting an input signal comprises modulating means for modulating the input signal, group delay deviation producing means for producing a group delay deviation on the modulated input signal, harmonic producing means placed on the downstream side of the modulating means for producing a harmonic of the modulated input signal to superimpose the harmonic on the modulated input signal, and harmonic phase shifting means placed on the downstream side of the modulating means for shifting a phase of the harmonic from the harmonic producing means to compensate the group delay deviation.
Still furthermore, according to the present invention, in a signal transmission apparatus, a receive section for receiving a signal comprises demodulating means for demodulating the signal, group delay deviation producing means placed on the upstream side of the demodulating means for producing a group delay deviation on the signal, harmonic producing means placed on the upstream side of the demodulating means for producing a harmonic of the modulated input signal to superimpose the harmonic on the signal, and harmonic phase shifting means placed on the upstream side of the demodulating means for shifting a phase of the harmonic from the harmonic producing means to compensate the group delay deviation.
Moreover, according to the present invention, in a signal transmission apparatus for transmitting an input signal comprises modulating means for modulating the input signal, group delay deviation producing means for producing a group delay deviation on the modulated input signal, harmonic producing means placed on the downstream side of the modulating means for producing a harmonic of the modulated input signal to superimpose the harmonic on the modulated input signal, and harmonic level changing means placed on the downstream side of the modulating means for changing a level of the harmonic from the harmonic producing means to compensate the group delay deviation.
Still moreover, according to the present invention, in a signal transmission apparatus, a receive section for receiving a signal comprises demodulating means for demodulating the signal, group delay deviation producing means placed on the upstream side of the demodulating means for producing a group delay deviation on the signal, harmonic producing means placed on the upstream side of the demodulating means for producing a harmonic of the modulated input signal to superimpose the harmonic on the signal, and harmonic level changing means placed on the upstream side of the demodulating means for changing a level of the harmonic from the harmonic producing means to compensate the group delay deviation.
In addition, according to the present invention, a signal transmission apparatus comprises modulating means for modulating a first signal into a second signal, demodulating means for demodulating the second signal into the original first signal, group delay deviation producing means placed between the modulating means and the demodulating means for producing a group delay deviation on the second signal, and harmonic phase shifting means placed on the upstream side of the demodulating means for shifting a phase of the second signal to compensate the group delay deviation.
Still additionally, according to the present invention, a signal transmission apparatus comprises modulating means for modulating a first signal into a second signal, demodulating means for demodulating the second signal into the original first signal, group delay deviation producing means placed between the modulating means and the demodulating means for producing a group delay deviation on the second signal, and harmonic level changing means placed on the upstream side of the demodulating means for changing a level of the second signal to compensate the group delay deviation.